gguf-py: byteswapping improvements (#12851)
* gguf-py: implement byteswapping for Q4_0
This is needed to byteswap Mistral model.
Also restore original shapes after byteswapping tensors.
It is not needed at the moment, but do it in case
they'd be used in future.
* Rework byteswapping code in gguf-py
Move out details from byteswapping tensor blocks code
